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Aerodynamics Group

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Efficiency is reduced by the need for ATC to separate aircraft to avoid conflicts then merge again for landing

ATC Sectors Los Angeles, USA

ATC Sectors in South East England, UK

Need to React with ATC which must Separate Aircraft then Merge to Land

ATC often have to take aircraft away from their optimum route and altitude to separate aircraft safely.

The distance between aircraft, and loss of efficiency, depends upon the navigational accuracy of the total system – could vary from 3 miles under radar to 120 miles en-route. Latest navigation systems can reduce en-route space to 5 miles.

ATC is split into separate centres, sometimes determined nationally, and liaison between centres can reduce efficiency.

During descent aircraft may have to cross points between centres at specific altitudes thus flying level rather than following an efficient continuous descent with idle thrust.

Need to React with ATC which must Separate Aircraft then Merge to Land

As aircraft approach their destination, ATC must merge aircraft into a stream to the runway to achieve the most efficient landing rate.

At present this is usually achieved by ATC giving aircraft headings and speeds to fly at low levels which stretch the approach path while aircraft are placed in sequence at the required spacing for the type causing extra fuel consumption and noise over the ground.

New Air Traffic Management Systems will merge aircraft into their landing sequence earlier in the flight, and allow more efficient descents with idle thrust leading to quieter Constant Descent Approaches with no periods of level flight.

The complexity of the process to merge traffic efficiently can be seen from the aircraft tracks into Schiphol airport at Amsterdam and simulations of the Paris arrival routes.

Simulation of Paris Arrivals

Maximize Take-off Weight

Prime Requirement –

Sound Knowledge of Take-off Performance Principles

To Take-off at the Maximum Allowable Weight for the Conditions

Carry Minimum Safe Fuel Reserves to Maximize Payload

Cost of Extra Fuel which can reduce Payload

On long sectors extra fuel displaces payload thus losing revenue - loss is extreme on very long sectors when the tanks are already full and the only way to fly further is to reduce the passenger load/weight.

Cost of carrying Extra Fuel – Not Restricting Payload

Burnt at app 3% per hour

Carrying extra fuel over the minimum flight plan fuel always involves as penalty due to the extra weight burnt at 3% per hour.

The actual cost of extra fuel depends on the relative cost of fuel between the departure and destination airfields.

If the fuel is cheap enough at the departure airfield it can be worthwhile carrying/tankering extra fuel into the destination.

However the effect of the extra weight on the aircraft must be considered – extra landing distance, possible extra brake wear and use of reverse thrust, reduced maximum cruise altitude, etc.

This decision is best made by the crew on the day who need to know the cost of extra fuel for the most economic judgement. For example:

Many companies do not publish Fuel Price Differentials but just tell crews when to “tanker” fuel, which may not be efficient.

Engine Out Altitudes may only be available on graphs

Cruise Speed & Fuel Consumption Relationship

Cruise – Crews Need to be Aware of Aircraft Performance

Crews should be have a good knowledge of the performance of their aircraft such as:

Optimum speeds for minimum cost, minimum fuel, etc and the penalties for flying away from the normal/recommended speeds.

Maximum altitudes for the aircraft weight and air temperature – All engines (provided by the FMS) and if limited by engine thrust of airframe buffet (not shown by FMS). Engine(s) Out altitude which may not be shown by the FMS with all engines running and be only available from graphs which are difficult to read quickly.

Table of Airbus A320 All Engines and Engine Out information – easier to access than FMS. All Engines Max Altitude is always limited by Climb Thrust.Available after FMS failure.

Correct Descent from Cruise Altitude Essential

Crews can still have to calculate/monitor descent mentally

Descent – Large opportunity for Fuel Savings – or Wastage

Reduction of True Air Speed at Low Altitude at the same Indicated Air Speed causes increase in fuel consumption and flight time

Descending early wastes fuel and time, can expose aircraft to icing conditions and more aircraft traffic, makes more noise, etc

Descent – Large opportunity for Fuel Savings – or Wastage

THERE IS NO TRADE BETWEEN FUEL & TIME DUE TO A POOR DESCENT

Summary of Penalties Cause by Poorly Executed Descents:

(Written in 1973 – some of us were worried about the environment then….)

Descent – Large opportunity for Fuel Savings – or Wastage

Circular slide rule primarily designed to help crews follow an efficient flight idle descent profile to comply with an ATC clearance such as to cross 23 DME XYZ at 8,000ft at 250kts.

Direct DME-Altitude checks are available throughout to verify on the profile. A fixed gradient of 400ft per mile above 10,000ft is suitable for IAS of 300-340kts according to aircraft weight, and 300ft per below 10,000ft for 250kts IAS after an 8 mile nm deceleration.

Checking the profile mentally, normally by 300ft per mile, requires regular computation of an equation, such as at 50 DME:

(50-8-23) x 300 = 5,700 + 8,000 = 13,700ft

In a survey BOAC B747 pilots estimated their efficiency was improved by at least 10 miles when using the computer, covering the cost of the 2 provided on each aircraft in 1 flight.

Besides minimising fuel burn and noise, following this profile improves safety by keeping the aircraft well clear of the ground into nearly all airfields.

Descent – Large opportunity for Fuel Savings – or Wastage

United Airlines nearly bought the circular computer but while the fixed gradient was suitable 747s & DC10s, DC9s found it too steep and B727s too shallow for their high speeds.

This linear computer has the altitude and sink rate on an elastic scale which can set gradients from 250ft per mile for slow speed descents or when in a tailwind up to 600ft per mile suitable for high speeds on a light aircraft into headwinds of 200kts. Could provide smoother descents than A340 FMGEC but not worth the effort for reduced engine changes.

Aircraft FMS now fly efficient descents, but if taken off the planned route by ATC pilots can be back to calculating the best profile using mental arithmetic.

7. Approach – Critical for Fuel Savings & Noise Reduction

Approach is the phase of flight after descent when the aircraft is decelerated and configured by extending flaps for the final approach.

Ideally it is a short period of continuous descent.

ATC may need to give headings and speeds while aircraft are merged into a landing stream, when flaps and landing gear must be extended as late as possible to minimise the extra fuel burnt.

The baseline of the table giving comparative fuel consumption is when cruising at FL370/37,000ft.

Minimum fuel is consumed while descending which shows that long slow descents with idle thrust are the most fuel efficient.

Maximum noise and fuel consumption, 400% more than at cruise altitude, is when flying level with flaps and gear extended (500% on a B747), but reduced when descending on the final glidepath even with the extra drag of full landing flap.

This demonstrates that level flight should be resisted if possible and that level flight with flaps and gear extended should avoided at all costs.

Baseline Cruising at 37,000ft

Maximum Fuel Consumption

Minimum Fuel Consumption

7. Approach – Critical for Fuel Savings & Noise Reduction

This shows that city life need not be disturbed significantly if aircraft are flown level with minimum flap setting above 3,000ft, preferably at least 5,000ft, before descending on the glideslope to the runway with gear up until about 1,500ft to be established for landing by 1,000ft. (On Airbus aircraft the gear can be extended at 800ft, like the Space Shuttle, but this is not the approved procedure.)

7. Approach – Critical for Fuel Savings & Noise Reduction

One operator into London Heathrow required the flaps and gear extension to be confirmed in the Initial Approach Checklist which was completed when leaving the entry points to the London area, so the aircraft could fly with the gear extended for up to 60 miles.

With the extra drag of the gear and flaps the aircraft would descend steeply and then fly at low altitude across central London making conversation impossible when over flying.

Aircraft noise disturbance over central London was a significant factor in the 1971 decision that the third London airport should be built 100km East of London on the Essex/North Sea coast, but this project was terminated after the 1973-4 fuel crisis.

7. Approach – Critical for Fuel Savings & Noise Reduction

To try and reduce the extreme levels of noise over central London this article was published in the GAPAN Journal of March 1974 (Appendix A in the CEAS paper and at www.Dibley.eu.com.) Suggesting that crews should ideally fly a continuous descent from the entry point to intercept the runway glideslope and extend the landing at about 1,500ft to be stabilised in the landing configuration by 1,000ft.

The idea was accepted by UK NATS and after input from Lufthansa who were proposing their similar Managed Drag Procedure, Constant Descent Approaches were started into LHR in 1975. DMEs were installed to give crews continuous distance to the runway paid for by the Department of Trade who was responsible for Noise Abatement.

However CDAs into LHR were not implemented as well as hoped as the procedure has yet to be included in the manufacturers operating manuals. While local operators are proficient less regular visitors will tend to descent early to intercept the glideslope from below.

Similar CDAs can be flown into airports like JFK - immediately reducing noise on the approach.

7. Approach – Critical for Fuel Savings & Noise Reduction

The type of CDA introduced into London and the Netherlands can give worthwhile noise reductions from 10 to 25 miles from the runway with no additional technology, and are being implemented in other airports such as Sacramento.

However at busy airports merging aircraft into an efficient sequence for the approach can be more difficult with aircraft trying to fly CDAs.

Future ATM systems due in service by about 2010 will allow efficient CDAs from cruise altitude, but procedures using parts of this system are already operating in some areas as described later.

UPS have been integrating their own aircraft flying CDAs into Louisville, which is possible because UPS is the only operator there at night.

Similarly because of their relatively low level of traffic the Swedish aviation authority LFV have been developing “Green” 4D trajectories flying CDAs into Stockholm Arlanda, both locally from and across the Atlantic.

However crews can still make savings using their own initiative.

8. Crews Can Save Fuel/Time by Choosing Approach/Runway

Approach tracks into busy airports can be structured with a long lead in for bad weather, and some are flown automatically to follow agreed noise routes.

When traffic and weather permits, crews should be allowed to fly shorter visual approaches

Past Examples of Operational Fuel Savings

Example of 8% Immediate Fuel Saving by Crews

Flight data recording showed that an aircraft fleet was not operating efficiently.

A fuel economy newsletter listed the flight segments and what how much extra fuel was being burnt / could be saved by a better operation.

The total extra burn was possibly 26% but this was unlikely to be saved as not all items would occur on one leg.

After crews were made aware of the penalties and some changes in procedures an 8% saving was achieved immediately.

Departure/arrival procedures in italics are not optimised in current operations.

1979 prices

Potential Fuel Saving 26%

Crew Fuel Monitoring Graphs

Top – Cost of Extra Fuel Uplifted

Centre – Cost of Extra Fuel Burnt

Bottom – Crew’sTotal Extra Cost

Past Examples of Operational Fuel Savings

A contract was secured because the crews’ more efficient operation saved 13% fuel compared to the previous operator which covered the crews’ cost.